This invention relates to a process for preparing a hollow, rib-reinforced, laminated article.
A general process for making laminated articles by first making a laminate preform from two thermoplastic sheets, then heating the preform, placing the heated preform between two mold halves, and blow-forming the article by introducing a fluid between the sheets to expand the sheets into their respective mold cavities, is well known in the art.
When the sheets are laminated to form a preform, however, they stick together and are very difficult to separate without tearing. This tendency of the sheets to stick together creates problems in introducing the blowing fluid between the sheets and in sheet separation as the blowing fluid expands the sheets into the mold cavity.
U.S. Pat. No. 3,281,301 discloses a method typical of the prior art wherein certain preselected areas of the interfacial surface between two laminated thermoplastic sheets are treated prior to lamination in such a manner as to make them nonadherent to each other. This can be accomplished by using a parting membrane such as paper or a layer of talc.
In addition, a bleeder member, such as a tube, is inserted between the sheets and is used to introduce air between the sheets. Air forced in between the sheets through the bleeder member follows the path progressively through the nonadherent areas distending the sheets in such areas.
In similar processes, see U.S. Pat. Nos. 3,271,846; 3,289,281; and 3,394,446 as the process appies particularly to thermoforming metal sheets.
These prior art processes have the disadvantage of requiring the use and placement of a nonadherent masking material on the sheets, which necessitates a separate operation and precludes subsequent sealing of the sheets where the masking material has been placed. Moreover, the laminated sheets must be precisely registered in the mold in order that the nonadherent area conform with the recessed portions of the mold, and a bleeder member must be properly aligned with and inserted between the laminated sheets for the introduction of air.
Other related prior art includes processes wherein two thermoplastic webs are heated and fed either continuously or incrementally between two advancing opposed mold halves which have an opening for the insertion of a blow tube. The thermoplastic webs, however, are not laminated prior to or during forming but instead are spaced apart to aid in introducing air between the sheets.
In U.S. Pat. No. 3,537,138, for example, the advancing webs are spaced apart and brought together only at points forming the edge of the final article and at all times leaving an opening, such as the mouth of a bottle being formed, where air can be introduced between the sheets to blow the sheets apart and into their respective mold cavities.
In still another related patent, namely, U.S. Pat. No. 3,099,043, the advancing webs are spaced apart and brought into contact with two opposing and partially opened mold halves. The webs, respectively, are vacuum drawn into their corresponding mold half and thereafter the mold halves each containing half of a formed article are brought together fusing the edges of the article forming the whole article while constantly maintaining communication between the atmosphere and the interior of the article being formed.
While many prior art processes are known for thermoforming plastic materials, there is a continuing need for a process having a convenient method for introducing air between the thermoplastic sheets. The need is especially apparent where the thermoformed article has an intricate design, such as a hollow rib-reinforced laminated article.